A. Field of the Invention
The present invention relates generally to the field of semiconductor devices and more specifically to high frequency devices designed to operate at medium and high RF power levels.
B. Prior Art
Modern technology has increased the demand for semiconductor devices which are capable of providing a high power output at high radio frequencies, HF through UHF. However, it is well known that as the output power level of high frequency power transistors increases, the impedance at the input terminals of the power transistor decreases. This has necessitated the use of impedance transforming networks between the power transistor and the source of RF energy, i.e. and RF generator or the collector circuit of another power transistor. Impedance transforming is desirable because it results in the maximum amount of power transferred from the energy source to the power transistor. Since the energy source generally has an output impedance of about 50 ohms, the power transistor input impedance should be matched for maximum power to be transferred from the source to the power transistor.
It is well known that for frequencies up to approximately 100 MHz, matching networks external to the power transistor are used for impedance transformation. However, at operating frequencies above this, the parasitic effects of the bond wires become substantial, and must be included in the overall impedance transforming network. This requires that a portion of the matching network be included within the transistor package. Microwave techniques for impedance transformation within the package of the power transistor have been shown in U.S. Pat. No. 3,713,006 which describes an input transforming network, and U.S. Pat. No. 3,969,752 which describes an output compensating network.
The prior art shown in U.S. Pat. No. 3,713,006 includes the insertion of a MOS capacitor next to the transistor die within the package. The MOS capacitor forms a matching network in conjunction with the inductance created by the base bond wires or leads which connect the base metallic conductor or lead of the package with the base bonding terminals of the transistor die. This inductance-capacitance combination transforms the input impedance from a low impedance to a higher impedance. In this way, the prior art accomplishes an increase in the input impedance of the power transistor.
A limitation of the prior art method of increasing input impedance has been that it requires the addition of a MOS capacitor onto the transistor header which substantially increases not only the package fabrication time but the package fabrication cost.
An object of the present invention is to provide transformation by the use of bond wires acting as shunt inductances.